FIG. 1 is a block diagram showing the configuration of a conventional optical wavelength division multiplexing and transmission apparatus. In FIG. 1, 101 indicates a transmitter end rack. 102 indicates a rack of a receiver end. The transmitter end rack 101 accommodates an optical wavelength multiplexer 111 and an amplifier 112. A plurality of input optical wavelength signals are multiplexed in the optical wavelength multiplexer 111, and an output of the optical wavelength multiplexer 111 is optically-amplified in the amplifier 112. The receiver end rack 102 accommodates an amplifier 114 and an optical wavelength demultiplexer 115. A multiplexed signal is transmitted and optically-amplified in the amplifier 114, and an output of the amplifier 114 is demultiplexed in the optical wavelength demultiplexer 115 to a plurality of optical wavelength signals having wavelengths different from each other. 113 indicates an optical transmission line formed of an optical fiber which connects the transmitter end rack 101 and the receiver end rack 102. A plurality of optical relaying units are normally placed at appropriate positions of the optical transmission line 113. However, the optical relaying units are omitted in this specification to simplify the description.
Next, an operation will be described below.
In a wavelength division multiplexing and transmission system (hereinafter, called WDM transmission system), a large number of types of information are respectively assigned to a plurality of optical wavelength signals having wavelengths different from each other to convert the types of information into the optical wavelength signals. In the transmitter end rack 101, the converted optical wavelength signals λ1, λ2, λ3, . . . and λn are taken out from the optical wavelength multiplexer 111 as a multiplexed signal. This multiplexed signal is amplified in the amplifier 112 and is sent out to the optical transmission line 113. In the receiver end rack 102, the multiplexed signal transmitted through the optical transmission line 113 is amplified in the amplifier 114 and is demultiplexed to the optical wavelength signals λ1, λ2, λ3, . . . and λn having wavelengths different from each other in the optical wavelength demultiplexer 115. The optical wavelength signals λ1, λ2, λ3, . . . and λn taken out from the optical wavelength demultiplexer 115 are demodulated in a latter stage to a plurality of electric signals denoting the types of original information.
Because the conventional optical wavelength division multiplexing and transmission apparatus has the above-described configuration, it is not easy to expand a multiplex function so as to increase the number of optical wavelength signals. A plurality of wavelengths of a plurality of optical wavelength signals multiplexed in an optical wavelength multiplexer are set to be placed at wavelength intervals so as not to interfere with each other. Therefore, in cases where the number of optical wavelength signals multiplexed with each other is increased due to the increase of a quantity of information to be transmitted, wavelengths assigned to pieces of information and possible to be processed in both an optical wavelength multiplexer and an optical wavelength demultiplexer run short. In this case, it is required to replace both the optical wavelength multiplexer and the optical wavelength demultiplexer with both another optical wavelength multiplexer and another optical wavelength demultiplexer in which additional optical wavelength signals can be received and processed. However, because both the optical wavelength multiplexer and the optical wavelength demultiplexer set in operation are taken out from the conventional optical wavelength division multiplexing and transmission apparatus, the communication is interrupted during the replacement. Therefore, a problem has arisen that it is not easy to actually perform the replacement.
The present invention is provided to solve the above-described problem, and the object of the present invention is to provide an optical wavelength division multiplexing and transmission apparatus in which both an optical wavelength multiplexer and an optical wavelength demultiplexer for an early operation are installed at an processing capacity corresponding to a prescribed number of optical wavelength signals to suppress an initial cost and a plurality of groups of other optical wavelength multiplexers and other optical wavelength demultiplexers suitable for the demand of communication are additionally installed one after another while maintaining a communication condition.